This invention relates to a slip-controlled hydraulic brake system with a braking pressure generator, which is pressureless in its basic position and which is connected to a storage reservoir. The system further includes at least one wheel brake cylinder for generating a braking torque at the driven wheels and a brake like for connecting the braking pressure generator with the wheel brake cylinder. A first check valve is connected into the brake line and a relief line is connected to the wheel brake cylinders. A second check valve is connected into the relief line. A pressure medium pump also is included having a first connection and a second connection, with the pressure medium being delivered from one connection to the other and with the first connection being connected to the relief line and with the second connection being connected to the braking pressure generator.
A brake system of this general type is described in U.S. Pat. No. 4,743,075 wherein wheel braking pressure control is achieved by letting pressure medium off from the wheel brakes of a wheel threatened by a lock-up and passing it to a storage reservoir so as to redeliver it into the wheel brakes by means of a pump for reaccelerating the wheel. Connected into each of the pressure medium paths to the storage reservoir and to the pump is a check valve for opening or closing the respective line depending upon the demands of the control device for adjusting an optimum brake slip. The check valve provided in the line leading to the pump can be considered an inlet valve and the check valve provided in the line leading to the storage reservoir can be considered an outlet valve. Further, a control valve is connected into the line leading to the pump. For adjusting the pressure at the outlet of the pump to a value proportional to the foot pressure.
For controlling traction slip pressure, medium is supplied to the wheel brake cylinders of the driven wheels in order to generate a braking torque contrary to the driving torque. In this way, the driving wheels will be prevented from spinning during starting or when driving on slippery ground. In order to prevent overcompensation of the driving torque it is possible to let pressure medium off from the wheel brakes. During traction slip control, pressure medium is supplied to the wheel brakes out of the pump, the braking pressure generator being avoided in this action. To this end, further valve means are provided that will prevent the pressure medium from returning out of the pump into the braking pressure generator which will be connected to the storage reservoir when it is not actuated.
Alternatively, it has been suggested to supply the pressure medium out of the pump to the wheel brakes via the relief line and not to supply it by bypassing the braking pressure generator. Thereby, the check valve in the relief line performs the function of an inlet valve while the check valve in the line leading to the pump, or rather to the braking pressure generator, adopts the function of an outlet valve. In this manner, the design of the brake system becomes simpler. Nonetheless, this system still requires valve means connecting the relief line alternatively to the storage reservoir or to the pump outlet. It is, therefore, an object of this invention to further simplify the brake system and, if possible, to completely dispense with any additional valve means.